Add support for Markdown rendering Homepage content

[shank4804]

This PR adds support to render the homepage contents from a markdown file called homepage.md if it is present. Currently, the only way the homepage contents are set from homeText parameter set in the config.toml file. With this new addition, whatever is written in the homepage.md gets rendered easily.

I found this by using the solution posted in the main Hugo repo: gohugoio/hugo#330 (comment)

[shank4804]

@zwbetz-gh Can you review this? I am currently using it this way in my website. Just wanted to push it up since someone else might run into the issues. :-)

[zwbetz-gh]

Hi there, thanks for the PR.

The Netlify build is currently failing with this error:

Error building site: failed to render pages: render of "home" failed: "/opt/build/repo/layouts/index.html:10:7": execute of template failed: template: index.html:10:7: executing "main" at : wrong number of args for isset: want 2 got 1

[zwbetz-gh]

@shank4804 I like this idea, and I've implemented it in 82aa487.

Checkout my changes to index.html, you may want to copy that syntax for your site, it is more terse :)

I'll go ahead and close this PR. Thanks again